Dynamic population codes for perception PROJECT SUMMARY One of the most critical functions of sensory and motor systems is preparation for upcoming events. In sensory systems, this preparation confers a perceptual and behavioral advantage. When the location of an upcoming event is known, subjects respond with shorter reaction times and greater accuracy ? an internal state that is often referred to as spatial attention. Numerous regions throughout the cortex have been implicated as playing important roles in establishing and maintaining attention, making it clear that attention operates through the coordinated activity of neuronal populations throughout the brain. Typical studies of attention, however, have focused largely on averaged neuronal responses in a time window well after the time of stimulus onset, and nearly all have involved recordings from a single brain region at a time. We will investigate the population-level mechanisms by which neurons prepare for an anticipated stimulus and maintain an attentional state, focusing on the activity of neurons within visual and prefrontal cortex as well as the interactions between these regions. Our strategy is to employ population-level measures to reveal signals hidden from single-neuron and averaged approaches, and then link these measures to behavior. In the first specific aim, we measure how neurons in visual cortex prepare for an upcoming stimulus. We hypothesize that a diverse population code underlies attentional preparation in visual cortex and is reflected in the earliest responses. In the second specific aim, we determine how prefrontal cortex and visual cortex work in concert to prepare attention. We hypothesize that prefrontal cortex serves to maintain a stable attentional state in the absence of visual stimulation, and coordinated activity between these regions influences behavior. In the third specific aim, we seek to understand how preparation in visual cortex is adaptable based on context. We hypothesize that dynamic task demands influence the patterns by which visual cortex prepares, enabling attention to flexibly influence behavior in numerous situations. The overall result of this study will be to establish the role of population activity in dynamic visual perception, and to construct a framework by which to relate population recordings in multiple brain regions to visual perception and behavior. This will aid in developing treatments for neurological disorders of vision and rehabilitation after traumatic brain injury or disease.